1. Field of the Invention
The invention relates to semiconductor wafer processing and, more particularly, the invention relates to optical emission spectroscopy for endpoint detection.
2. Description of the Background Art
Semiconductor processing systems employ numerous methods for monitoring the progression of processes that take place within a process chamber. Optical emission spectroscopy (OES) has been used in the prior art for monitoring and analyzing the characteristics of a plasma within the process chamber during ionizing (i.e., plasma) processes such as dry etch. An OES system analyzes light emitted by the plasma to determine the chemical composition of the plasma. The plasma generally contains byproducts of the process taking place within the chamber. Some of these byproducts are excited by the plasma and emit radiation in the form of infrared (IR), visible or ultraviolet (UV) light. Analysis of the light emitted by the excited byproducts reveals information about the process taking place within the chamber.
A typical OES apparatus uses, for example, a monochromator that is coupled to a transparent viewing window of the reaction chamber. Light generated by the plasma is carried by an optical fiber to the monochromator, and the monochromator selects a particular wavelength for analysis using a diffraction grating. The particular wavelength is disbursed from the grating at a specific angle to a photomultiplier detector or some other form of light detector. The photomultiplier detector (PMD) or other form of photon detector produces an electrical voltage representing the magnitude of energy at the particular wavelength selected by the monochromator. This voltage is typically analyzed by a computer system to detect the end point of the plasma enhanced etch process. Such OES systems have many uses in analyzing, characterizing and otherwise monitoring a plasma within a reaction chamber of a semiconductor processing system. Such OES systems are disclosed in U.S. Pat. No. 5,288,367 issued Feb. 22, 1994; U.S. Pat. No. 5,308,414 issued May 3, 1994; and U.S. Pat. No. 4,859,277 issued Aug. 22, 1989.
OES has also been used for determining the endpoint of processes such as photoresist stripping. In photoresist stripping, a layer of photoresist on a partially completed semiconductor circuit is removed by chemical reaction with reactive species from a plasma. The chemical reaction produces byproducts that enter the plasma. The byproducts are excited by interaction with the plasma and produce radiation. An OES system monitors the radiation for changes that indicate the endpoint of photoresist stripping. This technique is valuable in processes where the stripping chemistries emit a useable spectra for OES based process monitoring. Such chemistries exist, for example, in a microwave downstream reactor.
Unfortunately, several new chemistries used in photoresist strip processes "quench" the useable spectra as the strip process progresses and thus make it impossible to analyze the process by OES. Additionally, some wafer processing steps do not use plasma; i.e., they are non-ionizing processes. These non-ionizing processes cannot be monitored by OES.
Therefore, a need exists in the art for a method and apparatus for monitoring semiconductor processes that use chemistries that quench useable spectra. Furthermore, a need exists for a method and apparatus suitable for monitoring non-ionizing processes using OES techniques.